Circular single-knit jersey fabrics are used to make underwear and top-weight garments, such as T-shirts. Compared to woven structures, the knit fabric can more easily deform, or stretch, by compressing or elongating the individual knit stitches (comprised of interconnected loops) that form the knit fabric. This ability to stretch by stitch rearrangement adds to the wearing comfort of garments made from knit fabrics. Even when knit fabrics are constructed of 100% hard yarns, such as cotton, polyester, nylon, acrylics or wool, for example, there is some recovery of the knit stitches to original dimensions after imposed forces are removed. However, this recovery by knit stitch rearrangement generally is not complete because hard yarns, which are not elastomeric, do not provide a recovery force to rearrange the knit stitches. As a consequence, single-knit fabrics may experience permanent deformations or ‘bagging’ in certain garment areas, such as at the elbows of shirtsleeves, where more stretching occurs.
To improve the recovery performance of circular, single-knit fabrics, it is now common to co-knit a small amount of spandex fiber with the companion hard yarn. If heat setting is not used to “set” the spandex, after the fabric is knitted and released from the constraints of the circular knitting machine, the stretched spandex in the fabric will retract to compress the fabric stitches so that the fabric is reduced in dimensions compared to what those dimensions would be if the spandex were not present.
Heat setting is not used for all varieties of weft knit elastic fabrics. In some cases a heavy knit will be desired, such as in double knits/ribs and flat sweater knits. In these cases, some stitch compression by the spandex is acceptable. In other cases, the bare spandex fiber is covered with natural or synthetic fibers in a core-spinning or spindle-covering operation, so that the recovery of the spandex and resultant stitch compression is restrained by the covering. In still other cases, bare or covered spandex is plated only on every second or third knit course, thereby limiting the total recovery forces that compress the knit stitches. In seamless knitting, a process wherein tubular knits are shaped for direct use while being knitted on special machines, the fabric is not heat set because dense, stretchy fabrics are intended. For circular-knit jersey elastic fabrics made for cutting and sewing, however, wherein bare spandex is plated in every course, heat setting is almost always required. Heat setting has disadvantages. Heat setting is an extra cost to finish knit elastic fabrics that contain spandex, versus fabrics that are not elastic (rigid fabrics). Moreover, high spandex heat setting temperatures can adversely affect sensitive companion hard yarns, e.g., yellowing of cotton, thereby requiring more aggressive subsequent finishing operations, such as bleaching. Aggressive bleaching can negatively affect fabric tactile properties, such as “hand,” and usually requires the manufacturer to include fabric softener to counteract bleaching. Also, heat-sensitive hard yarns, such as those from polyacryonitrile, wool and acetate, cannot be used in high-temperature spandex heat setting steps, because the high heat setting temperatures will adversely affect such heat-sensitive yarns.
The disadvantages of heat setting have long been recognized, and, as a result, spandex compositions that heat set at somewhat lower temperatures have been identified (U.S. Pat. Nos. 5,948,875 and 6,472,494 B2). For example, the spandex defined in U.S. Pat. No. 6,472,494 B2 has a heat set efficiency greater than or equal to 85% at approximately 175 to 190° C. The heat set efficiency value of 85% is considered a minimum value for effective heat setting. It is measured by laboratory tests comparing the length of stretched spandex before and after heat setting to the before-stretched spandex length. While such lower heat setting spandex compositions provide an improvement, heat setting is still required, and the costs associated with it have not been significantly reduced.
The traditional practice of making and heat setting circular-knit fabrics has further disadvantages. The knit fabric emerges from a circular knitting machine in the form of a continuous tube. As the tube is formed in knitting, it is either rolled under tension onto a mandrel, or it is collected as a flat tube under the knitting machine by plaiting or loose folding. In either case, the fabric establishes two permanent creases where the fabric tube has been folded or flattened. Although the fabric is “opened” by slitting the fabric tube along one of the creases, subsequent use and cutting of the fabric usually must avoid the remaining crease. This reduces the fabric yield (or the amount of knit fabric that can be further processed into garments).
Recent advances in this area include U.S. Pat. No. 6,776,014, which describes the formation of circular knit fabrics suitable for t-shirts. In U.S. Pat. No. 6,776,014, elastified circular knit fabrics are knit using low draft and as a result, there is no heat setting required to achieve stable fabrics. However, the fabrics of U.S. Pat. No. 6,776,014 must be knit at very low spandex yarn tension in order to achieve stable fabrics.